This invention relates to fully differential voltage controlled transconductors, and more particularly relates to such transconductors that operate at low voltage with highly linear performance and wide input and control ranges.
Voltage controlled transconductors are used in a wide range of applications, for example in active filters circuits-such as Gm-C filters-multipliers and oscillators. Detailed descriptions of their construction and principles of operation are readily available. For example, see Analog Integrated Circuit Design, by David A. Johns and Ken Martin, John Wiley and Sons, New York(copyright) 1997, pages 605-607. FIG. 1 shows a representative prior art voltage controlled transconductor (VCT) circuit that is based on the linear mode of operation of MOSFETs. As is known, linear mode based transconductors provide better performance than saturation mode transconductors. The circuit comprises transistors M1 through M9 and operational amplifiers 101, 102 and 103, interconnected as shown. The circuit provides differential output currents Io+ and Ioxe2x88x92 in response to a differential input voltage Vid which is superimposed on a constant common-mode voltage signal Vcm. The drain-to-source voltages (Vds) of input transistors M1 and M2 are held to a control voltage Vc to control the current flowing into transistors M1 and M2, thereby controlling the transconductance of the entire circuit.
In applications where high precision is required, a VCT circuit like that of FIG. 1 can provide less than adequate performance in certain key areas. For example, such a circuit may not be truly fully differential. In other words, the absolute values of gm+ and gmxe2x88x92 may not be exactly the same. In addition, such a circuit suffers from nonlinearities in currents Io+ and Ioxe2x88x92. That is, the values of gm+ and gmxe2x88x92 are functions of Vid, instead of being constants. These nonlinearities and that the circuit is not truly differential can be understood because the theoretical circuit operation is based on the ideal behavior of MOSFET transistors in the linear mode, which does not take into account the second order effects and the non-idealities.
Another problem is that the range of the control voltage Vc may be insufficient to provide an adequately robust and controlled performance. This is because in the circuit of FIG. 1 if a wide input range is needed then the control voltage range must decrease in order to keep transistors M1 and M2 in the linear mode of operation, and vice versa, which is a disadvantage at low supply voltages.
Finally, the VCT circuit of FIG. 1 requires a third operational amplifier 103, for the common mode input Vcm. The common mode itself must be extracted from the input signal, which needs an extra circuit, as well.
It is therefore desirable to have a VCT circuit that overcomes some or all of the above-described problems.
In accordance with the present invention there is provided a voltage controlled transconductor (VCT) for receiving a differential input signal comprising a first voltage signal and a second voltage signal, and for providing a differential output signal comprising a first output current signal and a second output current signal. The VCT includes a first side transconductor circuit having two parts of the same construction, the first part being capable of conducting a first current signal and the second part being capable of conducting a second current signal, the first current signal and the second current signal being controlled by the first voltage signal. The VCT also includes a second side transconductor circuit having two parts of the same construction, the first part being capable of conducting a third current signal and the second part being capable of conducting a fourth current signal, the third current signal and the fourth current signal being controlled by the second voltage signal. A first control circuit is adapted to control the first and second current signals of the first side transconductor circuit, while a second control circuit is adapted to control the third and fourth current signals of the second side transconductor circuit. The first side transconductor circuit is connected to the second side transconductor circuit to provide the first output current signal comprising a difference of the second current signal and the third current signal and to provide the second output current signal comprising a difference of the first current signal and the fourth current signal.